A qualitative explanation of these data is possible. The spectrometers o n the M A C electron microprobe analyzer are fully focusing as shown in Figure 1. The source, the diffracting crystal, and the detector all lie in a Rowland circle. As lower 20 settings are selected, the crystal moves toward the sample along a straight line and rotates about its center. Thus, at the lower 28 settings, where precision is relatively poor, the crystal is very close to the source. Therefore, a n incremental change in the z sample placement results in a larger 20 error as compared to the error if the spectrometer were set at a much higher 20 value. F o r the M A C instrument, a change in the sample placement, AZ, along the optical axis produces a change of approximately
in the measured wavelength (d is the crystal spacing in angstroms, 0 is the correct reflected angle, and Z is expressed in microns). This simplified equation explains the qualitative characteristics of Figure 4 which shows that the precision is poor at the lower 20 settings. Furthermore, if the d spacing is relatively large as is the case for the K A P crystal, a given AZ increment results in a relatively large increment in diffracted wavelength. Diffracting crystals with the largest d spacings
should therefore produce the poorest precision, this effect is illustrated by the data plotted in Figures 2 and 3. More experimental and theoretical work is required in order t o fully characterize the precision of electron microprobe analysis. Yet this study has definitely demonstrated that there are major sources of scatter in electron microprobe data other than X-ray intensity fluctuations but that a careful evaluation of these sources of scatter can show how to avoid a loss in precision. It has been shown that the ulitmate precision available (that determined by the photon emission statistics) can be obtained in the M A C electron microprobe analyzer for many analyses of homogeneous metals. This can be accomplished by positioning the specimen through the 0.33 N.A. microscope objective and by chosing X-ray lines for monitoring which allow optimum 20 spectrometer settings. Of course, many sources of scatter which have not been considered here are inherent in routine analytical work. The sample preparation, surface contamination, inhomogeneities in the sample, shifting position of the point of electron impact, and sample charging can all cause poor precision. Since the samples used in this study were pure elemental metals, these other causes were isolated from the effect of specimen repositioning. RECEIVED for review May 1,1968. Accepted August 20, 1968
Determination of Trace Elements in Organic Material by the Oxygen Bomb Method Shizuo Fujiwara Department of Chemistry, Faculty of Science, The Uniaersity of Tokyo, Hongo, Tokyo
Hisatake Narasaki Department of Chemistry, Faculty of Science and Engineering, Saitama University, Shimoskubo, Urawa
SEVERAL METHODS have been developed for determining trace elements in organic substances. Dry or wet ashing procedures are the most simple, and have been most widely used. However, they include serious sources of error; for example, with the dry method one cannot prevent the loss of some elements by volatilization, and with the wet method, contamination often results from concentration of impurities in the digesting reagents employed. The present paper describes a method of ashing in which a n oxygen bomb is used. The advantages of this method are: the sample is decomposed in a closed system, the procedure of decomposition is simple and rapid, and contamination originating in the digesting reagents is eliminated.
in the bomb was brought up to 25 kg/cm2, and the samples were ignited by passing a small ac current through the platinum wire coil under a potential difference of 10 volts. The combustion products from 1 gram of polyethylene were examined in a preliminary study. Analysis of both the liquid and vapor phases in the bomb was made before and after combustion. Determination of the components in the gas phase was made by mass spectrometry. Nitric and nitrous acids in aqueous solutions produced after combustion (as a result of using liquid-air oxygen) were determined by ultraviolet spectrometery ( I ) . (1) H. Hamaguchi, R. Kuroda, and S. Endo, Bunseki Kagaku, 7, 409 (1958).
EXPERIMENTAL
The construction of the bomb, which is made of 18-8 Cr-Ni stainless steel, is shown in Figure 1. The capacity is 300 ml. [The values have been chosen for the convenience of the bomb used in this experiment. They are intermediate of the values of bombs in the literature (Appendix).] The electrodes and the combustion cup are of platinum. Use of fused silica is an alternative. As will be explained later, it is recommended that the interior of the bomb be platinumplated. Analytical samples were tied u p in a sheet of rice paper by means of cotton thread and put in the cup. Then the remaining parts of the thread were passed through a coil of platinum wire between the electrodes. Oxygen pressure
Table I. Analyses of Combustion Gas of Polyethylene in an Oxygen Bomb by Mass Spectrometry( % v/v) Mean value of two determinations Oxygen Gases in the bomb supplied Before ignition After ignition 0 2 99.02 95,84 68.76 N2 0.77 3.77 1.99 Ar 0.21 0.15 0.29 coz 0.02 28.95 co 0.15
VOL. 40, NO. 13, NOVEMBER 1968
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Table 11. Recoveries of Various Elements Treatment after ignition Precipitate: KHSO4 fusion Filtrate: digested with HzS04 (1 1) None
+
+
Spectrophotometry Oxine-benzene extraction
Recovery, 80
Mercuric thiocyanate-ferric perchlorate Neocuproine-CC11 extraction 2,2 ’-Bipyridyl Dithizone-CClr extraction Dithizone-CCla extraction Phosphovanadomolybdate Yellow molybdosilicic acid method Tiron
90-95
Digested with aqua regia (1 1) 95-91 95-91 Digested with HzS04 “01 96-100 None 93-95 Digested with HNOB(1 1) 100 Digested with ”01 HClOa 90 Precipitate: Na~C03fusion 80 Precipitate: KHSOa Filtrate: digested with HzSOI (1 1) N-benzoylphenyl hydroxylamine 92-96 Digested with HClO4 Dithizone-CCIa extraction 90 Digested with “ 0 3 (1 1) hough acid addition to complete the analysis for these e--ments has the disadvantage of inviting contamination, it is recommended for good recovery. Such contamination is negligible.
+
+
+
+
+
In order to investigate the recoveries of various elements, control analyses were carried out using filter papers that had been loaded with test portions of those elements. RESULTS AND DISCUSSION
Table I shows the results of the gas analysis. The oxygen used in this experiment contained a small amount of nitrogen, because it was obtained from air. The nitrogen content in the gas phase in the bomb is about five times that of the original oxygen. This is caused by the air which was left in the bomb in the process of filling with oxygen. Upon ignition of the sample, the chemical components forming the sample enter both the liquid and gas phases inside the bomb, although they tend to concentrate in the liquid. The liquid phase, where nitric and nitrous acids are determinable, is acidic. When the air is evacuated from the bomb before filling with oxygen, the contents of nitric and nitrous acids are found to be reduced to about one fifth of what they are when combustion is performed without previously removing the air. Hence it is assumed that these acids are formed in the main from nitrogen in the air present in the bomb. In either case, with or without evacuation of the air in the bomb, 5 mole of nitrogen is converted. Recoveries of the individual elements are summarized in Table 11. I t is evident that recoveries of chlorine and mercury are good when they are determined immediately after ignition. I n the cases of copper, iron, lead, vanadium, and zinc, recoveries are not complete unless the liquid produced by the combustion is digested with strong acids (shown in Table 11). This procedure is recommended for ionization of the elements and to ensure sensitivity of a given element to its color reagent. Phosphorus, for example, is changed to polymetaphosphate upon ignition; for this element the digestion procedure is necessary for conversion to orthophosphate ions, I n the case of aluminum o r titanium compounds, (2) S . Fujiwara and H. Narasaki, ANAL.CHEM.,36, 206 (1964). (3) A. Tomonari, Nippon Kagaku Zasski, 83, 693 (1962). (4) A. R. Gahler, ANAL.CHEM., 26, 577 (1954). (5) E. B. Sandell, “Colorimetric Determination of Traces of Metals,” Interscience, New York, 1959. (6) N. A. Talvitie, E. Perez, and D. P. Illustre, ANAL.CHEM.,34, 866 (1962). (7) D. F. Boltz, “Colorimetric Determination of Nonmetals,” Interscience, New York, 1958. (8) D. E. Ryan, Aizalyst, 85, 569 (1960). (9) J. H. Yoe and A. R. Armstrong, ANAL.CHEM.,19,100 (1947).
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ANALYTICAL CHEMISTRY
L J U Figure 1. Schematic figure of oxygen bomb Electrodes (b) Coil (c) Sample wrapped with rice paper ( d ) Combustion capsule
(a)
the combustion products must be fused with potassium bisulfate; in the case of silicon, with sodium carbonate. This procedure is necessary because such elements have already been converted to their oxides by any nitrogen oxides present. Without the above modifications, recovery of these elements is low, and an interference exists for their determination by the oxygen bomb method.
Appendix Typical Analyses with the Oxygen Bomb Element to be deterSample species mined Dust As Tobacco Coffee Cocoa Gasoline Synthetic sample B including boron acetate n-Heptyl bromide Br C
c1
Benzoic acid Cystine Dextrose Lubricating oil Grease Coal
Amount taken, grams 0.5 -1 .o
1 .o
Bomb Capacity, Pressure, Absorbent kg/cm2 ml None 380 20 -30
340 400
0.6 -0.8
25
10 ml of water
30
20 ml of 2.5% NaHC03
191
None
4.011
0.3
...
Benzoic acid Dextrose Anthraquinone Cholesterol Paper
0.01
23.7
1 .o
360
25 -30
0
Coal Benzoic acid Naphthalene
0.10
3 27
7
P
Phosphorus sulfide Tricresyl phosphate Piston deposits Nucleoproteins Methionine Plant leaves Corn Animal tissues and secretions Polyethylene
0.05 0.05 0.5 0.30 0.003
500
35
340 50
45
Hg
S
Se
Ti
0.1
27 -40 25
Col: Bromothymol Blue and mannitol
( 12)
Vol: titrates with 0.05N
(13)
3.5 -4.0
Gas chromatography Carrier gas: oxygen
5 ml of 5 % Na2C03 5 ml of water
White oil
None
0.05 gram of CUO
15 ml of 0.1N KMn04 & 10 ml of 10% HzSOa 0 . 2 ml of water 0.1 gram of iodine 10 ml of 5% NaOH
None
NH4N03& decalin
...
30
2 ml of 0.05N “,OH 10-15 ml of water
...
...
...
-0.2 1.5
(11)
3.85 None
300 -500
H
Methanol
Method of determination Collected in a cloud chamber, and Gutzeit method
Reference number
Ind: KzCr04
0.008 1.0
F
Combustion acid None
(14)
Grav: determined as AgCl Col: Alizarin complexon, sodium carbonate fusion of the ash Gas chromatography Carrier gas : helium
(15) (16)
Compares the pressure change with that of another bomb including sucrose
(19)
Nephelometry : BaS04
(21)
...
Fluorometry: 3,3‘-diaminobenzidine
(22)
...
Col.: H?On Ash is fused by KHSO4
(23)
...
..
(17)
KNO, Na20z
Abbreviations: Grav. : gravimetric; Vol. : Volumetric; Ind. : indicator; Col.: colorimetry. This appendix has been prepared by H. Narasaki at the Saitama University, the assistances of M. R. Steffenson of the Parr Instrument CO. and Miss Yumiko Sugita of the Saitama University is gratefully acknowledged.
Mizuike and coworkers (10) have checked the recoveries of various elements by loading the filter paper with the appropriate radioactivz tracer. This study showed that when the ignition proceeded in the usual manner, most of the elements as well as aluminum, titanium, and silicon remain in the ignition cup. I t was noticed, however, that 20% of an element, at most, might be transferred to the wall of the bomb or to the liquid phase. Therefore it is recommended that the interior of the bomb be platinum-plated, so that all elements may be collected by rinsing the bomb out with dilute acid.
RECEIVED for review April 4,1968.
Accepted July 1,1968.
(10) A. Mizuike, Y .Iida, Y. Ujihira, and Y .Takada, “Instrumental Analysis of High Polymers,” Vol. I1 (in Japanese) Minami, Hirano, Araki, and Fujiwara, Eds., Hirokawa Publ. Co., Tokyo, 1964, p 169.
(11) F. P. Carey, G. Blodgett, and H. S. Satterlee, IND.ENG. CHEM.,ANAL.ED., 6, 327 (1934). (12) J. J. Bailey and D. G. Gehring, ANAL.CHEW., 33,1760 (1961). (13) M. S. Agruss, G. W. Ayers, Jr., and H. Schindler, IND.ENG. CHEM., ANAL.ED., 13, 69 (1941). (14) A. M. Vogel and J. J. Quatrone, Jr., ANAL.CHEM.,32, 1754 (1960). (15) Am. SOC.Testing Materials, D 808-63 (1966). (16) R. A. Durie and H. N. S. Schafter, Fuel, 43, 31 (1964). (17) Y. Mashiko, H. Konosu, and T. Morii, Kogyo KagakuZusshi, 67, 555 (1964); C.A., 61, 7703d (1964). (18) L. G. Borchardt and B. L. Browning, Tuppi, 41,669 (1958). (19) J. W. Whitaker, R. N. Chakravorty, and A. K. Ghosh, J. Sci. Ind. Research (Zudia), 15B, 72 (1956). (20) A. L. Conrad, Mikrochemie Mikrocliim. Acta, 38, 514 (1951). (21) G. Toennies and B. Bakay, ANAL.CHEM.,25, 160 (1953). (22) W. B. Dye, E. Bretthauer, H. J. Seim, and C. Blincoe, ibid., 35, 1687 (1963). (23) R. A. Anduze, ibid., 29, 90 (1957). VOL. 40, NO. 13, NOVEMBER 1968
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